Processes designed for the production of a high purity product stream at high recovery from gaseous feed streams by combining single or multi-stage membrane systems and multi-bed PSA units are well-known. The use of stand-alone membrane units to produce a very high purity stream, i.e., greater than 99%, was found to be inefficient since large membrane areas and power demands were required in order to achieve this high purity at high recovery rates. PSA units, on the other hand, proved to be very efficient in producing high purity components from feed streams containing the desired component at concentrations greater than 50 mole %, but become less efficient for treating relatively low purity, less than 50% streams to yield a high purity product at high recovery rates.
Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and light hydrocarbons.
Membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates.
A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other component of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component or components and a stream which is depleted in the selectively permeating component or components. The stream which is depleted in the selectively permeating component or components is enriched in the relatively non-permeating component or components. A relatively non-permeating component permeates more slowly through the membrane than at least one other component of the mixture. An appropriate membrane material is chosen for the mixture so that some degree of separation can be achieved.
Membranes for hydrogen separation have been fabricated from a wide variety of polymeric materials, including cellulose esters, polyimides, polyaramides and polysulfones. An ideal gas separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high separation factor (selectivity) and high gas permeability. Normally polymers possessing high separation factors have low gas permeabilities, and vice-versa.
In any case it is very uneconomical to purify a permeating desired component by membranes only to high purity levels higher than 99%.
On the other hand, a process using pressure swing adsorption or PSA is an adequate tool for separating and purifying hydrogen gas contained in a gaseous mixture with impurities which are selectively adsorbed by one or more adsorbing beds in a PSA system. Adsorption in these beds occurs at a more elevated pressure, the impurities which are more selectively adsorbed being desorbed through pressure reduction at a lower desorption pressure. It is possible to purge the beds at this lower desorption pressure for desorption and further withdrawal of impurities, before repressurization at higher adsorption pressure for adsorption of impurities of further amounts of the feed gaseous mixture during the working out of the process cycle.
As practised in the technique, the PSA process is commercially very important for the purification of hydrogen gas. The purity of hydrogen can be higher than 99.99%. The PSA process can be used to treat a wide variety of available feed streams and is not limited to a specific stream. There are no pre- or post treatment requirements, except for the withdrawal of impurities in order to avoid unduly adsorbent degradation. Further, there is no practically reduction in pressure from the feed stream and the product gas so that the product gas is available at the pressure level for further use downstream of the PSA system and for repressurization of each bed up to the adsorption pressure.
Pressure of hydrogen-containing streams submitted to separation through PSA reach 40 bar (600 psig) or higher.
The number of adsorbing beds in PSA systems vary widely, from two to 12 beds, on a case-to-case basis.
Thus, U.S. Pat. No. 4,398,926 teaches a process for the selective permeation using membranes combined to PSA where the technique of selective permeation separates a major amount of the impurities of the desired component which is contained in a gaseous feed stream at high pressure and relatively impure, final separation and purification being effected in a PSA system to which the membrane system is integrated. It is alleged that in this way the recovery of the product is improved, without sacrificing the purity, while the overall cost for producing the product is reduced. The process is said to be specially useful to treat high pressure streams, that is, having pressures higher than 40 bar (600 psig). It should be understood that the PSA systems show drawbacks when operating at high pressures of adsorption, that is, above 40 bar. At these high pressures the PSA system becomes very expensive in terms of investment costs. The trend is that a higher amount of product be retained in the bed at these high pressures, so that the desired component, for example hydrogen, will be discarded from the bed together with the impurities during the counter-current depressurization step. Thus U.S. Pat. No. 4,398,926 provides an optimized PSA process for the treatment of gaseous streams at high pressures. Thus, a portion of the excess pressure is used in the membrane system for a preliminary purification of the desired component, for example, hydrogen. It should be understood that the stream to be treated according to the claimed process should present a minimum molar amount in hydrogen near to 40% in order that the process be applied. The usual hydrogen content of the stream to be treated is usually near 90 mole % or higher, the balance of the molar content being impurities. The process is to be applied to gaseous streams of various gases, provided that the stream shows a high pressure and contains a relatively high content in impurities besides the desired gaseous component.
In U.S. Pat. No. 4,398,926 a first separation is effected in a Prism separator which contains a permeable membrane. The membrane of the separator is able to selectively permeate hydrogen for a pressure of the feed gas of the separator of the order of 40 bar (600 psig) and up to 134 bar (2000 psig) or more. The separator possesses an inlet means for the high pressure gas and and outlet means for withdrawing permeate gas enriched in hydrogen at a lower pressure. A further outlet means is provided in order to withdraw the non-permeate portion or retentate, from the gaseous stream. Commercial membranes useful in the practice of U.S. Pat. No. 4,398,926 comprise hollow fibers, usually made of polysulfones covered by silicon rubber, mounted within the structure of the separator. In the Prism separators the hollow fibers are assembled in compact bundles in order to establish the large membrane area available for the path of the hydrogen being separated from impurities present in the feed gas.
U.S. Pat. No. 4,690,695 teaches an improved process for separating gases with special emphasis on obtaining a high purity product. This patent cites that the permeable membranes able to selectively permeate a component from a gaseous mixture have their use limited by the pressure differential maintained on opposite sides of the membrane, the passage of the more permeable component through the membrane being enhanced as the pressure differential across the membrane is increased. On the other hand, the pressure differential is limited by practical operational conditions such as the strength of the membrane itself, and the compression costs to be applied in the separations. Even if several membranes are used in series, it is not possible to obain high purity gas at high recovery levels. In the process taught in U.S. Pat. No. 4,690,695 the bulk separation is effected using membranes, the permeate gas being directed to a PSA system for separation and recovery of high purity product gas. The waste gas from PSA is compressed and combined to the feed gas which should be directed to the membrane, from which non permeate gas or retentate is withdrawn under high pressure. The described process is useful for refinery gases which are mixtures of hydrogen and methane, for example those having a hydrogen content around 40 mole %, refinery gases which are mixtures of helium and nitrogen or still separation of air from which an oxygen enriched stream is desired. The product obtained is 99.99+% pure.
U.S. Pat. No. 4,701,187 teaches a process for the separation and recovery of a component from a gaseous mixture. The gaseous feed mixture is first separated in a membrane unit separation in order to yield a gaseous stream enriched in the desired component. The concentrated gaseous stream is later on separated in an adsorption unit which contains an adsorbent which selectively adsorbs the non-desired gaseous components, thus yielding a purified product stream. The non-desired gaseous components are further desorbed and a purge stream from the adsorption unit containing non-desired gaseous components together with a portion of the desired components is recycled to the gaseous feed mixture. In the process described in U.S. Pat. No. 4,701,187 the membrane used has high selectivity and low permeability. The initial molar content of the desired component is high. The technique described in this patent, if applied to gaseous streams of low molar content and/or at low pressure in the desired component will yield less than optimum results, as regards the molar content of the desired component recovery and will demand high energy input.
U.S. Pat. No. 4,863,492 teaches an integrated membrane/PSA process which yields a mixed gaseous product of pre-set gas ratio which can be adjustably monitored and a second gaseous component of high purity. The permeated stream from the permeable membrane system is fed to the PSA unit and the purge gas from the PSA unit is compressed and mixed to the non-permeate stream or retentate in order to obtain the mixed product gas. The pressure of the permeate stream is monitored in order to set the gas ratio of the mixed product gas. The process is used to set the composition of synthesis gas (H.sub.2 :CO) to a desired ratio and does not apply to the purification of hydrogen.
European Patent application EP 0684066 A2 teaches a method of recovering a light element comprising hydrogen or helium from a high pressure feed stream. The feed stream comprises the light element in a concentration of less than 30% by volume and also hydrocarbons and trace heavy contaminants. The trace heavy contaminants are removed from the high pressure feed stream by adsorption in one or more beds of activated carbon and the feed stream is passed at high pressure through a membrane unit which allows permeation of the light element. The membrane unit produces a process stream enriched to somewhere above 40% in the light elements and a mass flow rate that is only a fraction of the mass flow rate of the feed stream. The process stream is then compressed and subjected to a pressure swing adsorption process utilizing one or more adsorbents to at least adsorb the hydrocarbons to produce a product stream highly enriched in the light element to 98% enriched or above. The energy demand for recompression is determined by the flow rate of the permeate stream.
German DE 4232496 A1 teaches solvent-resistant permeable membranes molded from solutions in non-toxic solvents of polyamideimides prepared from dicarboxylic aromatic acids and diamines, the membranes being useful for the separation/chemical conversion of gases.
U.S. Pat. No. 5,248,319 teaches blends of aromatic polyamides, polyimides and polyamide-imides having high permeabilities to hydrogen and helium. The selectivities presented in this patent have been determined for pure gases. As for comparison purposes selectivities have to be determined for actual gaseous mixtures, data of this patent cannot be used as standards.
In view of the fact that the open literature is mainly directed to the separation/recovery of gases at high pressures and/or high molar contents in the desired component--for example, 40, 50 or even 90 mole % content--the polymeric membranes usually employed are preferably of high selectivity, low permeability. Besides, the described processes are directed to gaseous streams at high pressures, typically 60-90 bar. Therefore the state-of-the-art publications do not mention adequate membranes for the separation/recovery of gases present at low molar contents and/or low pressures, neither there is any mention whatsoever of processes which make possible the separation/ recovery, in the industrial scale, of gases present at molar contents as low as 10-30% associated to pressures as low as 8 to 15 bar. Therefore is it concluded that the membranes and processes as described in the open literature, if applied to the separation/recovery of components at low molar contents/low pressures, will not reach the amounts of separation/recovery industrially and economically sought. Thus, in spite of certain references such as U.S. Pat. No. 4,701,187 mention that the gaseous stream to be treated may contain of from 20 mole % of the desired component, such a low molar content will yield, under the process conditions described in said patent, extremely low molar contents of the desired component, which in practice renders the process inapplicable to low pressure, low molar content conditions.
In general, literature emphasizes the preference of high selectivity, low permeability membranes. If one considers using the high selectivity membranes of the state-of-the-art processes for a stream containing the desired component at low or very low molar content, part of the membrane performance cannot be utilized because of lack of pressure ratio across the membrane in relation to the high membrane selectivity, thus demanding increased membrane area corresponding to loss in recovery rate, thus rendering the process uneconomical.
The refinery off gas stream effluent from the Fluid Catalytic Cracking (FCC) units constitutes a particular problem as regards the separation/recovery of the hydrogen since this stream contains hydrogen at low pressures (between 8 and 15 bar) and low molar contents (between 10 and 30 mole %). The literature neither teaches nor suggests a system of permeable membrane/PSA able to recover economically hydrogen from these streams of low molar content/low pressure in the desired component. This stems from the fact that the partial pressure/fugacity of the desired component is the main driving force of the separation process using membranes. In the case of refinery off gas from FCC units the low pressures and low molar contents render this driving force extremely limited. Thus, the processes available from known techniques if applied to the unfavorable conditions of such off gas streams are rendered economically unfeasible. The economical unfeasibility stems from the fact that the low pressures require compression to increase the pressure of the feed stream to be permeated through the membrane, the compression costs being excessively high. The most widely used membranes are membranes which present low permeability to hydrogen and high selectivity to hydrogen/methane, so that permeation through this kind of membrane requires the high pressures which avoid that such processes be applied to the separation of gases from streams where the desired component(s) are present in low pressure and/or low molar contents. Also, the use of multi-stage membrane units is prohibitive because of the energy demand for inter-stage recompression.
Thus, in spite of the variety of processes of the state-of-the-art designed to the separation of gases by combining permeable membranes/PSA, there is still the need for a process for the separation/recovery of gases which would be directed to the separation/recovery of desired components from gaseous mixtures where the desired component is present at low pressures and/or low molar contents, such process being effected at low energy demand and the desired component being of high purity (99.9 mol %). Such process is described and claimed in the present invention.